A closer look at the militants tearing Iraq apart. Boasting with the brutality. With the images we showed you at the top. Tonight, we're learning more about them. And the man who leads them. He spent years in U.S. Custody. Terry Moran is in northern Iraq tonight where the militants have already taken over. Reporter: They are fierce, battle-hardened fighters. Religious zealots. Merciless killers. But the jihadists of the islamic state of Iraq and Syria, Isis as they're known, they have a strategy and it's working. They use social media as a weapon. Many of the images in this story come from accounts associated with Isis and can't be verified. The pictures of mass executions. Iraqi soldiers in civilian clothes, taken prisoner, bound, packed into trucks, laid out on the ground and savagely executed, they struck terror across Iraq. But some Iraq kis actually support Isis. S sunis who feel oppressed. Half a million people fled this city of mosul, Iraq's second largest when the jihadists took it over. These people are willing to go back. They're going home. They tell us they have no place else to go. They say Isis is turning on the lights. Paying government workers. Trying to set up a state. Far more, as far as the eye can see, are still fleeing the rule of the jihadists. U.s. Leaders say Isis must be stopped. These are the vicious, the worst of the worst. Reporter: The leader of Isis Abu bakhar Al baghdadi was in U.S. Custody in Iraq for four years. When he was released in 2009, his parting words to U.S. Troops, I'll see you in new York. There is word tonight that Al baghdadi may have been injured in an air strike not far from him. Whatever may happen to him, his die bollically successful leadership already threatens to tear his country apart. Our thanks to you both. We'll have reporting throughout the night at abcnews.com and more more on "Good morning America."

This transcript has been automatically generated and may not be 100% accurate.

The crisis in Iraq spreads amid reports of about 2,000 Iranian troops joining the fight.

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